1. Field of the Invention
This invention relates to stabilized polyurethane foam. More specifically, the present invention is concerned with stabilizing polyurethane foam by using a combination of a phenothiazine with a co-stabilizer selected from the group consisting of certain dithiocarbamates, thiuramsulfides, mercaptobenzothiazole, phosphites, phosphonites, phenolic antioxidants, pentaerythritoltetrakis(beta-laurylthiopropionate), 4,4'-thiobis(2-t-butyl-5-methylphenol) and mixtures thereof.
2. Description of the Prior Art
Polyurethane foams have been prepared from polyols stabilized against oxidative degradation with combinations of sterically hindered phenols, diphenyl amines and a phenothiazine, for example, as taught in U.S. Pat. Nos. 4,265,783; 4,275,173; 4,010,211; and 4,363,745.
The use of carbamates in connection with polyurethanes is also known. U.S. Pat. No. 3,006,870 (Steinfatt et al) discloses a polyurethane containing 0.2-5.0 zinc di-lower alkyl dithiocarbamate for the protection against fungi and bacteria.
U.S. Pat. No. 3,182,037 (Nelson) discloses the use of 0.5-10 parts of dithiocarbamate and 5.0 parts or more of TiO.sub.2 in 100 parts polyurethane for the stabilization against ultraviolet degradation.
U.S. Pat. No. 3,255,129 (Ferrari) discloses the use of 0.0-10 parts of certain zinc dithiocarbamates used alone for the stabilization of polurethane elastomers.
U.S. Pat. No. 3,432,474 (Lombardi) discloses the use of metal dithiocarbamates as catalysts for curing a mixture of polyurethane prepolymer and a curative therefore, such as a polyol. 0.2-5 percent of catalyst is used based on the total ingredients employed. No stabilization is mentioned in this reference.
U.S. Pat. No. 3,978,011 discloses the stabilization of flame-retardant polyurethane foam using zinc dithiocarbamate or zinc monocarboxylate in the presence of zinc oxide, antimony oxide or chlorinated polymer. Zinc salt is present at 0.01-0.9 parts computed as metallic zinc per 100 parts of polyol used.
U.S. Pat. No. 4,356,274 (Sandler) discloses a rigid polyurethane foam containing 0.1-20 parts mercaptobenzothiazole, dithiocarbamates and the like.
Furthermore, the use of certain hindered phenols and amine antioxidants in polyols is disclosed in other references such as, U.S. Pat. Nos. 3,567,664; 3,637,865; 4,007,230; 4,021,385; 4,070,304; and 4,228,247.
There is a constant search in the art to find ways to decrease the quantity of antioxidant necessary to effectively stabilize the polyol and polyurethane against oxidative degradation and the foams prepared therefrom from scorching.
It has now been discovered that combinations of a phenothiazine with certain co-stabilizers, combinations not heretofore discovered by the art, realize polyols and polyurethanes with greatly improved stability against oxidative degradation at the same level of prior art antioxidants or for the same amount of prior art stability utilizing reduced levels of antioxidant.